The present invention relates to a filter, particularly an annular axial-flow filter that is useful to filtrate foreign matters from air or oil. The axial-flow filter of the present invention is compact and light weight, therefore useful in various kinds of vehicles. The subject of the present invention is in the method to mass produce axial-flow filters and the apparatus to make those filters.
Conventionally, an axial-flow filter is made in a very primitive way, i.e., they used a flexible base sheet made of many corrugations, and they applied a flat sheet of filter material onto the base sheet, and then folded the filter material along the corrugations of the base sheet by hand to finally obtain a corrugated sheet of filter material. The method is as readily understood very unstable and time consuming. Therefore it took, for example, two minutes to make an axial-flow filter which is about 300 mm in diameter, which shows very low productivity. Especially it was impossible to uniformly fold the corrugations since it relied on hand work of workers in the line. Therefore the filtration performance varies from one filter to another and the appearance of the resultant filter is poor, thus reducing the value of the merchandise. Aside from the method above explained, applicants are not informed of any other method to produce an axial-flow filter. It is conceivable that an automated process to make the filter is difficult, since it has to deal with soft or flexible filter material which is different from rigid and solid material like metal or hard plastic.